With conventional distributor ignition systems each cylinder receives an ignition signal only on the firing compression stroke of that cylinder.
With a double ended coil ignition system, in which spark plugs are connected to both ends of the secondary winding of the ignition coil, the one coil is supplying the ignition signal for two cylinders. As the firing or compression stroke of one of the cylinders will coincide with the exhaust stroke of the other cylinder and the coil produces ignition signals simultaneously at both cylinders, one cylinder will receive a "real" ignition signal when that cylinder is on its firing stroke while the other cylinder receives a "wasted" ignition signal on its exhaust stroke.
For the synchronisation of an engine analyser, it is necessary to generate a synchronising signal identifying the beginning of the true ignition sequence of a known cylinder, usually the first cylinder in the firing order.